When it is desired to secure a movable panel to a fixed member such as to an upright of an outer frame member, it is known to use a sliding bolt mounted to the panel, the bolt end being fittable behind or within a keeper carried by the frame, whereby to effect the securement. The keeper typically is a plate mounted over a recessed portion of the fixed member or an apertured body secured to the fixed member. The panel typically is a hinged panel which closes perpendicularly or nearly so to the fixed member and thus to the keeper. In use the bolt is slidable between (a) a holding or forwardly extended position in which the bolt end can fit within the recess or aperture provided by the keeper whereby to secure the panel in the closed condition, and (b) a rearwardly retracted or non-holding position in which the bolt end cannot engage the keeper whereby to allow the panel to move relative to the fixed member.
Usually the bolt is mounted in a bolt housing, and is slidable between guides carried by the bolt housing. Various types of bolt and bolt assembly are known, some of which are lockable in the holding position.
One type of bolt assembly has resilient bias means for the bolt, the resilient bias means conveniently being a compression spring or a leaf spring mounted in the bolt housing, the spring urging the bolt end towards the holding position, the bolt end having a chamfer; in the condition of use the chamfer faces the keeper as the panel is being closed. As the panel is moved to its closed condition the keeper forces back the bolt against the spring and into its non-holding position, until the bolt end is aligned with the recess or aperture whereupon the spring urges the bolt into its holding position; this action is often referred to as latching, and the spring as a latch spring, whilst the bolt assemblies of which the biassed and chamfer-ended bolts form a part, are referred to herein as latch units.
One common form of latch unit is designed to be fitted to the interior surface at the rim of an inwardly opening door, such as a front door of a domestic dwelling. For simplicity, the following disclosure will refer to a latch unit for such use (rim latch); however, the latch unit of the invention can also be morticed into a door or other panel, and can be fitted to windows and other movable panels. The keeper for such latch unit will usually be formed with an aperture, sized to receive the bolt end and needing only to be affixed to the door frame or other fixed member in order to provide in the holding position a self-contained embracement of the bolt end, and for simplicity also the following disclosure will assume such a keeper.
A latch unit typically has a bolt which can be moved rearwardly (retracted) by an actuator. The actuator will often be connected at one side to a "thumb-turn" (usually the side which will be to the inside of the building in use), the thumb-turn being rotated to move the bolt from its holding to its non-holding position. At its other side the actuator will often be connected to the barrel of a lock (usually the side which will be to the outside of the building in use), so that from that side it is necessary to insert a key to retract the bolt from its holding position.
Because the bolt of a latch unit would otherwise automatically enter the keeper ("latching" action) when the door is closed, it is common to require a device to be fitted which an occupier can operate in order to permit the force of the latch spring to be over-ridden. Such device will therefore permit the bolt to be retained in its non-holding position i.e. non-latching condition, so that the occupier for instance does not need to use a key or other operating means in order to regain entry into the premises. A means to over-ride the latch spring, and to maintain the bolt away from its holding position is often referred to as a "latch-back" device.